For many years the very high contrast photographic images needed in the graphic arts and printing industries were obtained by developing a `lith` emulsion (usually high in silver chloride content) in a hydroquinone, low sulphite, `lith` developer by the process known as infectious development. However, such low sulphite developers are inherently unstable and are particularly inappropriate for machine processing.
More recently, emulsions containing hydrazine nucleating agents have been used and processed in a high pH (about pH 11.5) developer with conventional amounts of sulphite, hydroquinone and possibly metol or a pyrazolidone. While such a process is better than the low sulphite lith process, the developer still has less sulphite than is optimal and a high pH requirement for it to function correctly. Such a solution is not as stable as is desirable. Additionally high pH solutions are environmentally undesirable because of the care needed in handling and disposing of their effluent.
A further improvement in this area is the introduction of a lower pH process (below pH 11), the use of hydrazides active at this low pH and the additional use of an amine "booster" as described in U.S. Pat. Nos. 4,269,929, 4,668,605 and 4,740,452. The hydrazides proposed for use in such materials are described, for example in U.S. Pat. Nos. 4,278,748, 4,031,127, 4,030,925 and 4,323,643 and in European Patent 0,333,435.
In most photographic materials the type and size of the silver halide grain determines the speed of the material while also affecting the covering power of the silver image formed therefrom. In general smaller sized grains provide higher density and covering power than larger ones. In some materials therefore, there has to be a balance struck between speed and covering power. In high contrast materials another balance between vigorous development and pepper fog (which occurs if development is too vigorous) needs to be achieved.
U.S. Pat. No. 5,185,232 describes a method of forming a high contrast (a .gamma. of about 8) by exposing in a camera, through the transparent support to obtain lateral reversal. Such a method of exposure has its own problems as explained in the specification. The material used contains two emulsion layers, the one closer to the support having a speed higher by 0.1 to 0.4 logE than the other one. Both layers are clearly latent image-forming under conditions of use.
U.S. Pat. No. 4 920 034 describes a high contrast photographic material containing emulsions of differing sensitivities. The stated object of this specification is to provide high photographic speed, good dot reproduction and yet retain freedom from pepper fog. While the emulsions contain a hydrazide, the system uses a low sulphite developer having a pH of 11.6. The two emulsion layers used are both spectrally sensitised and image forming as shown in FIGS. 1 and 2.
U.S. Pat. No. 4,746,593 also describes a high contrast material having two emulsion layers, one of them having a lower grain volume than the other. Again a high pH developer is required. The large grain volume emulsion is the image-forming emulsion and the small grain emulsion is preferably a primitive emulsion (not chemically sensitised). In the Examples the emulsion of higher grain volume is primitive, both the emulsions are silver chlorobromide (85% bromide) and the developer has a pH of 12.